Drive circuits for high voltage solid state switches are well known. Typically this type of circuitry requires separate power sources and is normally accomplished by opto-isolators or transformers to couple a driving signal from a signal source to the gating electrode of a solid state switch. These and other types of drive circuits for high voltage applications typically require rather large isolating transformers and are therefore space consuming and costly in nature.
It is desirable to provide a low cost, small, isolated arrangement for driving the gate circuit of high voltage semiconductor devices such as metal oxide field effect transistors. These field effect transistors have come into wide usage and it is desirable to drive line voltage units from a relatively low voltage control or drive circuit. The present invention accomplishes this purpose by the use of a very small pulse type transformer and is capable of being operated with very low frequency rectangular switching voltage wave forms. This is accomplished by providing some means to differentiate the leading and falling edges of the input or signal voltage. This differentiated signal is then fed into the primary of a step-up type of pulse transformer. The voltages are stepped up substantially, and are provided as a series of sharp driving pulses to a bidirectional voltage breakdown means made up of a pair of series connected zener diodes that are arranged in a back-to-back configuration to drive the gate of a field effect transistor. The field effect transistor has a source-drain circuit connected to a high voltage source and can be driven with the present arrangement from the relatively low voltage, and low frequency rectangular wave signal voltage. This is accomplished by breaking down the zener diodes to charge a capacitor (which might be the input capacitance of the field effect transistor). The driving signal first charges the capacitor to operate the field effect transistor in one mode, and then allows for the discharge of the capacitor on the reverse half cycle of the applied signal to discharge the capacitor and operate the field effect transistor in the second mode of operation.
The present invention requires only a very small, low cost pulse transformer which will store and transmit a pulse of only a few microseconds in width. This eliminates the prior art arrangements wherein the gate drive circuits did not use zener diodes and required large, wide-band pulse transformers for voltage isolation.